All forms of electromagnetic radiation, whether they're called waves or rays, are generated by particles in motion. In a radio signal, for example, negatively charged electrons quickly move back and forth in an antenna. This movement creates a field surrounding the antenna that has both an electric and magnetic component.

Electromagnetic radiation is carried by photons, massless particles that travel at the speed of light (300,000 kilometers per second in a vacuum). Every photon is characterized by wavelength (the distance from the crest of one wave to the crest of the next wave), by frequency (the number of wave cycles that pass by in a given period, measured in Hertz, which stands for cycles per second), and by the energy it carries (measured in electron volts).